Fix some typos. (#1598)

common/fuzzing/carbon.proto

@@ -401,7 +401,7 @@ message CompilationUnit {
   optional LibraryName package_statement = 1;
   optional bool is_api = 2;
   repeated Declaration declarations = 3;
-  // TODO Add support for imports if they are useful in fuzzing.
+  // TODO: Add support for imports if they are useful in fuzzing.
 }
 
 // Top-level fuzzer input.

explorer/ast/pattern.h

@@ -66,7 +66,7 @@ class Pattern : public AstNode {
   }
 
   // The value of this pattern. Cannot be called before typechecking.
-  // TODO rename to avoid confusion with BindingPattern::constant_value
+  // TODO: Rename to avoid confusion with BindingPattern::constant_value
   auto value() const -> const Value& { return **value_; }
 
   // Sets the value of this pattern. Can only be called once, during

third_party/examples/re2/re2.carbon

@@ -357,7 +357,7 @@ class RE2 {
     return f(sp, re, nullptr, 0);
   }
 
-  // TODO (variadics)
+  // TODO: (variadics)
   // TODO: template <typename F, typename SP, typename... A>
   // TODO: static inline bool Apply(F f, SP sp, const RE2& re, const A&... a) {
   // TODO:   const Arg* const args[] = {&a...};