fix typo in sf1.Rmd

vignettes/sf1.Rmd

@@ -570,7 +570,7 @@ functions are limited to PostGIS.
 Coordinate reference systems (CRS) are like measurement units
 for coordinates: they specify which location on Earth a particular
 coordinate pair refers to. We saw above that `sfc` objects (geometry
-list-columns) have a an attribute of class `crs` that stores the
+list-columns) have an attribute of class `crs` that stores the
 CRS. This implies that all geometries in a geometry list-column
 have the same CRS. It may be `NA` in case the CRS is unknown, or
 when we work with local coordinate systems (e.g. inside a building,
@@ -583,7 +583,7 @@ name (if existing) or the user-definition of the CRS, and `wkt`
 for the WKT-2 specification; WKT-2 is a standard encoding for
 describing CRS that is used throughout the spatial data science
 industry.  When defining a CRS, a PROJ string may be used that is
-understood by the [PROJ](https://proj4.org/) library. It defines
+understood by the [PROJ](https://proj.org/) library. It defines
 projection types and (often) defines parameter values for particular
 projections, and hence can cover an infinite amount of different
 projections. Alternatively, codes like `EPSG:3035` or `OGC:CRS84`
@@ -592,7 +592,7 @@ the PROJ database.
 
 Coordinate reference system transformations are carried out using
 `st_transform()`, e.g. converting longitudes/latitudes in NAD27 to
-web mercator (EPSG:3857) can be done by:
+Web Mercator (EPSG:3857) can be done by:
 
 ```{r}
 nc.web_mercator <- st_transform(nc, "EPSG:3857")