docs/mca.rst: fix MCA environment variable prefix for PRRTE

The table added in 061f908 (A variety of docs updates:, 2022-09-12)
mentioning the different prefixes for Open MPI, PMIx and PRRTE MCA
parameters set via environment variables has one too many "R"'s in
'PRRTE_MCA_': the correct prefix is 'PRTE_MCA_'. Fix that, and make it
clear that it is not a typo.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
(cherry picked from commit bd9adb4)

Author: phil-blain

docs/mca.rst

@@ -252,13 +252,13 @@ shells):
              When setting PMIx- and PRRTE-specific MCA parameters via
              environment variables, use a different prefix:
 
-             +----------+----------------+
-             | Open MPI | ``OMPI_MCA_``  |
-             +----------+----------------+
-             | PMIx     | ``PMIX_MCA_``  |
-             +----------+----------------+
-             | PRRTE    | ``PRRTE_MCA_`` |
-             +----------+----------------+
+             +----------+-----------------------------------+
+             | Open MPI | ``OMPI_MCA_``                     |
+             +----------+-----------------------------------+
+             | PMIx     | ``PMIX_MCA_``                     |
+             +----------+-----------------------------------+
+             | PRRTE    | ``PRTE_MCA_`` (with a single "R") |
+             +----------+-----------------------------------+
 
 Tuning MCA parameter files
 ^^^^^^^^^^^^^^^^^^^^^^^^^^