docs/develop/optimizations: Fix typo identified in issue 14391.

Signed-off-by: Matt Trentini <matt.trentini@gmail.com>

docs/develop/optimizations.rst

@@ -33,7 +33,7 @@ Variables
 MicroPython processes local and global variables differently. Global variables
 are stored and looked up from a global dictionary that is allocated on the heap
 (note that each module has its own separate dict, so separate namespace).
-Local variables on the other hand are are stored on the Python value stack, which may
+Local variables on the other hand are stored on the Python value stack, which may
 live on the C stack or on the heap.  They are accessed directly by their offset
 within the Python stack, which is more efficient than a global lookup in a dict.