When informing of a regex syntax let's do it with required separators in-place

If we inform the user about how to make a case sensitive search using a regex syntax, do it with the regex delimitators in place, so the user:

- Don't gets an error when copying and pasting the regex syntax to the github code search textbox. Error meaning it didn't find results because it lacks the regex slash separators.

- Avoid the user to have to also find in the documentation how to enter a regex in the code search textbox. Have to find out it needs to use the slash separators.

Author: nbenitez

content/search-github/github-code-search/understanding-github-code-search-syntax.md

@@ -317,4 +317,4 @@ If code search guesses wrong, you can always get the search you wanted by using
 
 ## Case sensitivity
 
-By default, code search is case-insensitive. Searching for `True` will include results for _uppercase_ `TRUE` and _lowercase_ `true`. You can do case-sensitive searches by using a regular expression with case insensitivity turned off, for example `(?-i)True`.
+By default, code search is case-insensitive. Searching for `True` will include results for _uppercase_ `TRUE` and _lowercase_ `true`. You can do case-sensitive searches by using a regular expression with case insensitivity turned off, for example `/(?-i)True/`.