I guess this was only a typographical error, where you inserted the variable 'T' into 'temperature', see below for the adjusted code:
n = 100; % gridpoints
T1 = 100; % temp T1
T2 = 0; % temp T2
%construct a tridiagonal matrix
A=2*eye(n);
A=A-diag(ones(n-1,1),1);
A=A-diag(ones(n-1,1),-1);
A(1,:)=0;
A(1,1)=1; %Since the temperature at node 1 is known
A(n,:)=0;
A(n,n)=1; %Since the temperature at node n is known
% Construct a vector b with known temperatures
b=zeros(n,1);
b(1)=T1;
b(end)=T2;
x = linspace(0,0.8,n);
Temperature = A\b;
T = Temperature(2:end-1); % extract the middle temperatues for the ex
figure
plot(x,Temperature)
title('Temperature distribution between x=0 and x=0.8 ')
xlabel('x')
ylabel('Temperature')
