Fix the dates for the C and C++ language working group

It's 1st and 3rd Wed of the month instead of every other.

Author: AaronBallman

llvm/docs/GettingInvolved.rst

@@ -216,7 +216,7 @@ writing, the following sync-ups are organized:
        `gcal <https://calendar.google.com/calendar/u/0?cid=ZDQyc3ZlajJmbjIzNG1jaTUybjFsdjA2dWNAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ>`__
      - `Minutes/docs <https://docs.google.com/document/d/1GahxppHJ7o1O_fn1Mbidu1DHEg7V2aOr92LXCtNV1_o/edit?usp=sharing>`__
    * - Clang C and C++ Language Working Group
-     - Every 2 weeks on Wednesday
+     - 1st and 3rd Wednesday of the month
      - `gcal <https://calendar.google.com/calendar/u/0?cid=cW1lZGg0ZXNpMnIyZDN2aTVydGVrdWF1YzRAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ>`__
      - `Minutes/docs <https://docs.google.com/document/d/1x5-RbOC6-jnI_NcJ9Dp4pSmGhhNe7lUevuWUIB46TeM/edit?usp=sharing>`__