Use if statement for post_build control flow

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

.readthedocs.yaml

@@ -15,8 +15,10 @@ build:
     - make apidoc
     - breathe-apidoc -o docs/api apidoc/xml
     post_build:
-    - '[ "$READTHEDOCS_VERSION" != "development" ] || "$READTHEDOCS_VIRTUALENV_PATH/bin/rtd" projects "Mbed TLS API" redirects sync --wet-run -f docs/redirects.yaml'
-
+    - |
+      if [ "$READTHEDOCS_VERSION" = "development" ]; then
+        "$READTHEDOCS_VIRTUALENV_PATH/bin/rtd" projects "Mbed TLS API" redirects sync --wet-run -f docs/redirects.yaml
+      fi
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: