Fix block string literals highlighting (#3635)

Replace block string literals quotes.

Highlighting after change:

![image](https://github.com/carbon-language/carbon-lang/assets/24532774/4c55c021-b8ce-4933-9742-81a8eee6ffe1)


Closes #3634

utils/textmate/Syntaxes/Carbon.plist

@@ -82,9 +82,9 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
                 <key>name</key>
                 <string>string.quoted.triple.carbon</string>
                 <key>begin</key>
-                <string>"""</string>
+                <string>'''</string>
                 <key>end</key>
-                <string>"""</string>
+                <string>'''</string>
 
                 <key>patterns</key>
                 <array>