Fix grammatical errors in explanation.md

Author: adelgadop

docs/explanation.md

@@ -2,22 +2,22 @@
 
 `siem` package helps you to create the emission files for WRF-Chem,
 and CMAQ air quality models.
-The strategy is to reproduces the model's netCDF emission files
+The strategy is to reproduce the model's netCDF emission files
 (same coordinates, dimensions, units, and attributes).
-As sometimes the original emission pre-processors requires very detailed information,
+As sometimes the original emission pre-processors require very detailed information,
 `siem` allows the user to create the emissions with limited information,
 and hence the _simplified_ in the name.
 Because of that, `siem` could be used in different cities to run air quality models,
 and, therefore, to improve air quality management.
 
 `siem` is strongly based on the methodology of Andrade et al. (2015),
-previously used in many air quality simulation over the Metropolitan Area of São Paulo.
+previously used in many air quality simulations over the Metropolitan Area of São Paulo.
 But, `siem` offers other features:
 
-- Adding different number of emission sources.
-- Point sources can be spatially and temporal distributed.
-- It is chemical mechanism agnostic. The chemical mechanism are not hard-coded.
+- Adding a different number of emission sources.
+- Point sources can be spatially and temporally distributed.
+- It is a chemical mechanism agnostic. The chemical mechanisms are not hard-coded.
 - Emission sources can vary by day of the week.
-- Each emission sources can have different spatial distribution.
+- Each emission source can have a different spatial distribution.
 - Early created for WRF-Chem, but now compatible with CMAQ.
-- As it is built in Python, subproducts can be easily modified (i.e. calculate total emissions, plot pollutant emissions, apply correction factors, etc).
+- As it is built in Python, subproducts can be easily modified (i.e., calculate total emissions, plot pollutant emissions, apply correction factors, etc).