Fix spelling and typographical errors found by cspell

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 9effa44..64a0bcc 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -315,7 +315,7 @@
 #if !defined(MBEDTLS_ECP_WINDOW_SIZE)
 /*
  * Maximum "window" size used for point multiplication.
- * Default: a point where higher memory usage yields disminishing performance
+ * Default: a point where higher memory usage yields diminishing performance
  *          returns.
  * Minimum value: 2. Maximum value: 7.
  *