Fix typo (#218)

Author: 一色双葉IsshikiFutaba

documentation/API.md

@@ -14,7 +14,7 @@ Processing routine. Should be periodically called form loop().
 bool addHreg(uint16_t offset, uint16_t value = 0, uint16_t numregs = 1);
 bool addCoil(uint16_t offset, bool value = false, uint16_t numregs = 1);
 bool addIsts(uint16_t offset, bool value = false, uint16_t numregs = 1);
-bool addIreg(uint16_t offset, uint16_t value = 0, uint16_t nemregs = 1);
+bool addIreg(uint16_t offset, uint16_t value = 0, uint16_t numregs = 1);
 ```
 
 ### Write local reg