Update usage.md

Fix typo:
synethetic -> synthetic

Author: pacharanero

docs/usage.md

@@ -92,7 +92,7 @@ nhs_number.generate(for_region=nhs_number.REGION_ENGLAND)
 # ['7709030025']
 ```
 
-!!! warning "Working with synethetic / unallocated numbers only"
+!!! warning "Working with synthetic / unallocated numbers only"
     Careless use of numbers generated from this library could result in data loss, damage or compromise. You should never use numbers generated by this library in queries against live health service systems. If you want to avoid generating numbers which may have been allocated to real patients, apply the `for_region=REGION_SYNTHETIC` parameter to constrain the output of the function to only numbers from the 900000000n - 999999999n testing range.
 
 ## `NhsNumber` object