Fix spelling of mbedtls_low_level_strerr

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/error-unification.txt b/ChangeLog.d/error-unification.txt
index bcf5ba1..eddd42c 100644
--- a/ChangeLog.d/error-unification.txt
+++ b/ChangeLog.d/error-unification.txt
@@ -7,5 +7,5 @@
      between -32767 and -1 as before.
 
 Removals
-   * Remove mbedtls_low_level_sterr() and mbedtls_high_level_strerr(),
+   * Remove mbedtls_low_level_strerr() and mbedtls_high_level_strerr(),
      since these concepts no longer exists. There is just mbedtls_strerror().
diff --git a/docs/4.0-migration-guide/error-codes.md b/docs/4.0-migration-guide/error-codes.md
index ca62025..8cc7098 100644
--- a/docs/4.0-migration-guide/error-codes.md
+++ b/docs/4.0-migration-guide/error-codes.md
@@ -12,7 +12,7 @@
 
 Generally, functions that used to return the sum of two error codes now return the low-level code. However, as before, the exact error code returned in a given scenario can change without notice unless the condition is specifically described in the function's documentation and no other condition is applicable.
 
-As a consequence, the functions `mbedtls_low_level_sterr()` and `mbedtls_high_level_strerr()` no longer exist.
+As a consequence, the functions `mbedtls_low_level_strerr()` and `mbedtls_high_level_strerr()` no longer exist.
 
 ### Removed error code names