Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 1cd6eb6..1da9802 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1329,7 +1329,7 @@
  * Include backtrace information with each allocated block.
  *
  * Requires: MBEDTLS_MEMORY_BUFFER_ALLOC_C
- *           GLIBC-compatible backtrace() an backtrace_symbols() support
+ *           GLIBC-compatible backtrace() and backtrace_symbols() support
  *
  * Uncomment this macro to include backtrace information
  */