PEP 831: Fix typo in JIT frame pointer explanation (#4919)

Author: brettcannon

peps/pep-0831.rst

@@ -411,7 +411,7 @@ code is a prerequisite for the JIT to be considered production-ready.
 
 Individual JIT stencils do not need frame-pointer prologues; the entire JIT
 region can be treated as a single frameless region for unwinding purposes.
-What matters is that the JIT itself is must reserve frame pointers, so
+What matters is that the JIT itself must reserve frame pointers, so
 that the frame-pointer register (``%rbp`` on x86-64, ``x29`` on AArch64) is
 reserved and not clobbered by stencil code.  With frame pointers in the
 JIT, most unwinders can walk through JIT regions without needing to inspect