[clang][docs] Fix documentation of -O

D79916 changed the behaviour from -O2 to -O1 but the documentation was
not updated to reflect this.

(cherry picked from commit 788c7d2)

Author: jrtc27

clang/docs/CommandGuide/clang.rst

@@ -385,7 +385,7 @@ Code Generation Options
     :option:`-Og` Like :option:`-O1`. In future versions, this option might
     disable different optimizations in order to improve debuggability.
 
-    :option:`-O` Equivalent to :option:`-O2`.
+    :option:`-O` Equivalent to :option:`-O1`.
 
     :option:`-O4` and higher