src/api: Fix speling issues

Found by codespell

src/api/netdb.c

@@ -62,7 +62,7 @@ int h_errno;
 #endif /* LWIP_DNS_API_DECLARE_H_ERRNO */
 
 /** LWIP_DNS_API_HOSTENT_STORAGE: if set to 0 (default), lwip_gethostbyname()
- * returns the same global variabe for all calls (in all threads).
+ * returns the same global variable for all calls (in all threads).
  * When set to 1, your port should provide a function
  *      struct hostent* sys_thread_hostent( struct hostent* h);
  * which have to do a copy of "h" and return a pointer ont the "per-thread"

src/api/tcpip.c

@@ -428,7 +428,7 @@ tcpip_untimeout(sys_timeout_handler h, void *arg)
 
 /**
  * Sends a message to TCPIP thread to call a function. Caller thread blocks on
- * on a provided semaphore, which ist NOT automatically signalled by TCPIP thread,
+ * on a provided semaphore, which is NOT automatically signalled by TCPIP thread,
  * this has to be done by the user.
  * It is recommended to use LWIP_TCPIP_CORE_LOCKING since this is the way
  * with least runtime overhead.