Changed regex example to use any github.com repo with regex search (#837)

Updated regex example to use another repo as sourcegraph/sourcegraph is
no longer available in .com instance

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---------

Co-authored-by: Maedah Batool <[email protected]>
Co-authored-by: Keegan Carruthers-Smith <[email protected]>

Author: 3 people

docs/code-search/queries/examples.mdx

@@ -107,8 +107,8 @@ repo:^github\.com/Parsely/pykafka$ Not leader for partition
 
 Regex searches are also useful when searching boundaries that are not delimited by code structures:
 
-[Finding css classes with word boundary regex](https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+%5Cbbtn-secondary%5Cb&patternType=regexp)
+[Finding css classes with word boundary regex](https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/+%5Cbbtn-secondary%5Cb&patternType=regexp)
 
 ```sgquery
-repo:^github\.com/sourcegraph/sourcegraph$ \bbtn-secondary\b
+repo:^github\.com/sourcegraph/ \bbtn-secondary\b
 ```