README: Polish wording

README.rst

@@ -229,8 +229,8 @@ limitations, please consider supporting the project (see above).
 
 * The IO subsystem is configured at compile time and there is no
   runtime fallback. For example, if io_uring is configured at compile
-  time, but not at runtime, SLASH/fellow will fail rather than falling
-  back to an alternative IO subsystem. (`#21`_)
+  time, but not available at runtime, SLASH/fellow will fail rather
+  than falling back to an alternative IO subsystem. (`#21`_)
 
 * SLASH/fellow data on persistent storage is stored in native byte
   order, so storage can not be transported to systems with a different