[3.6] Fix a build failure that occurs in environments using Code Page 950

[aphroteus]

Replace a non-ASCII character with a space

Description

A build error message was found:
ecdh.h(1): error C2220: the following warning is treated as an error
ecdh.h(1): warning C4819: The file contains a character that cannot be represented in the current code page (950). Save the file in Unicode format to prevent data loss

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line.
If the provided content is part of the present PR remove the # symbol.

• changelog provided | not required because: no change in functionality
• development PR provided # | not required because: file doesn't exist
• TF-PSA-Crypto PR provided Fix a build failure that occurs in environments using Code Page 950 TF-PSA-Crypto#411
• framework PR provided Mbed-TLS/mbedtls-framework# | not required
• 3.6 PR provided here
• tests provided | not required because: no change in functionality

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