Fix CRYPTO_add_lock documentation.

It's not actually CRYPTO_add_locked, despite the name. I guess they just needed
a name that didn't clash with CRYPTO_add.

Change-Id: I3fdee08bf75e9a4e1b5e75630707c0be5792599b
Reviewed-on: https://boringssl-review.googlesource.com/4102
Reviewed-by: Adam Langley <agl@google.com>