NFC: dts: st21nfca_i2c: Correct minor typo in documentation

Replaced st,st21nfca-i2c by st,st21nfca_i2c to be concistent
with below configuration and driver in drivers/nfc/st21nfca/
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/st21nfca.txt

 * STMicroelectronics SAS. ST21NFCA NFC Controller
 
 Required properties:
-- compatible: Should be "st,st21nfca-i2c".
+- compatible: Should be "st,st21nfca_i2c".
 - clock-frequency: I²C work frequency.
 - reg: address on the bus
 - interrupt-parent: phandle for the interrupt gpio controller