add missing imports for mean in docs example

Author: Red-Portal

README.md

@@ -95,6 +95,8 @@ y = Vector{Bool}(data[:, end] .== "Mine");
 Let's apply some basic pre-processing and add an intercept column:
 
 ```julia
+using Statistics
+
 X = (X .- mean(X; dims=2)) ./ std(X; dims=2)
 X = hcat(X, ones(size(X, 1)));
 ```

docs/src/tutorials/basic.md

@@ -89,6 +89,8 @@ nothing
 Let's apply some basic pre-processing and add an intercept column:
 
 ```@example basic
+using Statistics
+
 X = (X .- mean(X; dims=2)) ./ std(X; dims=2)
 X = hcat(X, ones(size(X, 1)))
 nothing

docs/src/tutorials/subsampling.md

@@ -87,6 +87,8 @@ The features start from the seoncd column, while the last column are the class l
 Let's also apply some basic pre-processing.
 
 ```@example subsampling
+using Statistics
+
 X = (X .- mean(X; dims=2)) ./ std(X; dims=2)
 X = hcat(X, ones(size(X, 1)))
 nothing