fix some typos (#1444)

Signed-off-by: cui fliter <imcusg@gmail.com>

explorer/fuzzing/ast_to_proto_test.cpp

@@ -97,7 +97,7 @@ auto GetUnusedFields(const Message& message) -> std::set<std::string> {
 }
 
 // A 'smoke' test to check that each field present in `carbon.proto` is set at
-// least once after converting all Carbon test sources to proto represention.
+// least once after converting all Carbon test sources to proto representation.
 TEST(AstToProtoTest, SetsAllProtoFields) {
   Carbon::Fuzzing::CompilationUnit merged_proto;
   for (const llvm::StringRef f : *carbon_files) {

explorer/testdata/pointer/heap_alloc_lifetime.carbon

@@ -11,7 +11,7 @@
 
 package ExplorerTest api;
 
-// The lifetime of a heap-allocated object is until it is explicitely
+// The lifetime of a heap-allocated object is until it is explicitly
 // deleted. Its lifetime is unrelated to the procedure call stack.
 
 fn AllocateInteger() -> i32* {

proposals/p0044.md

@@ -972,7 +972,7 @@ issues, the process is more light-weight without them.
 
 ### Rationale for pushing high-level comments to GitHub
 
-While opinions were not as strong, reasons given for perferring comments in
+While opinions were not as strong, reasons given for preferring comments in
 GitHub:
 
 -   This flow will maximize the alignment with “normal” GitHub development flow.

proposals/p0140.md

@@ -85,4 +85,4 @@ Disadvantages:
 
 Conversion of Markdown to another language at a later point (either manually or
 using a tool like Sphinx) is expected to remain a relatively low-cost option,
-due to the relative simplicitly of Markdown-formatted documents.
+due to the relative simplicity of Markdown-formatted documents.

proposals/p0555.md

@@ -185,7 +185,7 @@ operator to the other, precedences monotonically increase, so by transitivity of
 the precedence partial ordering, the ancestor operator has lower precedence than
 the descendent operator.
 
-An operator precedence parser with a partial ordering of predecence levels
+An operator precedence parser with a partial ordering of precedence levels
 [has been implemented](https://github.com/carbon-language/carbon-lang/commit/b8afadb3c6af5e68192d585232fee759180ea1e3)
 as a proof-of-concept in the Carbon toolchain.
 

toolchain/parser/parse_tree.h

@@ -328,7 +328,7 @@ class ParseTree::PostorderIterator
   Node node_;
 };
 
-// A forward iterator across the silbings at a particular level in the parse
+// A forward iterator across the siblings at a particular level in the parse
 // tree. It produces `ParseTree::Node` objects which are opaque handles and must
 // be used in conjunction with the `ParseTree` itself.
 //