Add blank line to correctly format.

reStructuredText requires blank lines to make a new paragraph. Otherwise
the text will be rendered as a continuation of the previous paragraph,
with no line break.

Author: Maetveis

llvm/docs/SPIRVUsage.rst

@@ -140,6 +140,7 @@ To enable multiple extensions, list them separated by comma. For example, to ena
 
 To enable all extensions, use the following option:
 ``-spirv-ext=all``
+
 To enable all KHR extensions, use the following option:
 ``-spirv-ext=khr``