Fix spellings

Author: cgzones

src/benchmark/relative_speed.rs

@@ -54,7 +54,7 @@ fn compute_relative_speeds<'a>(
             };
 
             // https://en.wikipedia.org/wiki/Propagation_of_uncertainty#Example_formulas
-            // Covariance asssumed to be 0, i.e. variables are assumed to be independent
+            // Covariance assumed to be 0, i.e. variables are assumed to be independent
             let ratio_stddev = match (result.stddev, reference.stddev) {
                 (Some(result_stddev), Some(fastest_stddev)) => Some(
                     ratio

src/cli.rs

@@ -155,7 +155,7 @@ fn build_command() -> Command {
                      string '{VAR}' in each command by the current parameter value.\n\n  \
                      Example:  hyperfine -P threads 1 8 'make -j {threads}'\n\n\
                      This performs benchmarks for 'make -j 1', 'make -j 2', …, 'make -j 8'.\n\n\
-                     To have the value increase following different patterns, use shell arithmetics.\n\n  \
+                     To have the value increase following different patterns, use shell arithmetic.\n\n  \
                      Example: hyperfine -P size 0 3 'sleep $((2**{size}))'\n\n\
                      This performs benchmarks with power of 2 increases: 'sleep 1', 'sleep 2', 'sleep 4', …\n\
                      The exact syntax may vary depending on your shell and OS."